Typically, automotive vehicles are provided with an engine cooling system including a heat exchanger, which is usually referred to as a radiator. When the engine is running, heat is transferred from the engine to a coolant that flows through the engine. The coolant then flows from the engine to the heat exchanger through a series of conduits. At the heat exchanger, heat is transferred from the coolant to cooler air that flows over the outside of the heat exchanger. This process repeats itself in a continuous cycle thereby cooling the engine.
Heat exchangers are also used in air conditioning systems for intercoolers in turbochargers and superchargers, and for auxiliary cooling of electronic power supplies in electric vehicles.
A typical heat exchanger includes a series of tubes supported by two chambers, which are usually called headers, positioned at either end of a heat exchanging portion, which is usually called the matrix. The matrix comprises a series of parallel tubes which carry a liquid coolant between the headers, on the way from an input port and an outlet port to the headers. Air flow between the tubes helps to dissipate heat in the cooling medium. To increase the surface area of the matrix and increase the ability of the matrix to dissipate heat, the tubes are usually spanned by a series of fins that extend either in parallel in a direction transverse to the length of the tubes, or in a zigzag orientation between the tubes.
Although the headers may be partly or wholly of a polymer material, the matrix of the heat exchanger is of metal, for example an aluminium alloy. The header has a base plate, also normally of metal, to which the ends of the tubes are connected. Side walls of the headers may be of metal, but for reasons of cost are now often made from a plastic material, which is secured to the metal base plate, for example by crimps in the metal, with a seal between the metal base plate and header side walls being made by a compliant, compressible gasket or o-ring that extends around the periphery of the join between the base plate and side walls.
There are two known ways of fabricating such heat exchangers. One is to use “controlled atmosphere brazing” (CAB) to bond together the matrix and metal part of each header that is joined to the matrix. Any such CAB process or welding process is referred to in this description as a “heating and fusing process”.
The other known way of fabricating such heat exchangers is to avoid welding or brazing of adjacent metal components by using “mechanical assembly” (MA) of the matrix and headers. In this description, the terms “mechanical joints” and “mechanically joined” are used to refer to any such non-welded or non-brazed joints in which adjacent components are held together mechanically by separate in-contact components that are not otherwise bonded together.
In the CAB process, flattened metal tubes are interspaced with metal fins that span the gaps between tubes, usually in a zigzag pattern. In many CAB heat exchangers, the tubes each comprise a single enclosed channel or, alternatively, a pair of side-by-side single channels that are separated by a longitudinally extending partition wall to form a double enclosed channel. The tubes have a generally elongate, substantially rectangular cross-sectional shape, and comprise two opposing, longer sides or faces that are substantially flat, and two opposing curved shorter sides, or ends. The fins are then brazed to the long sides and do not extend substantially beyond the bounds of the short sides. The ends of each tube extend inside apertures in metallic header base plates. The gap between adjacent metal components is kept to less than about 0.15 mm so that the gaps are spanned and sealed by solder when the assembly is passed through a braze furnace to form the braze joint between components. The metal components are preferably all of aluminium alloy to provide high thermal conductivity.
In the MA process, the fins, tubes and headers are all held together not by metallic joints but by friction or mechanical coupling. The fins, instead of being folded or corrugated to extend in the same direction as the tubes, extend continuously at right angles to the tubes, and therefore have apertures through which each tube passes. In this arrangement, the fins are closely spaced apart in parallel, and usually extend to the opposite front and rear surfaces of the matrix. The tubes have a circular cross-section and initially have a diameter less than the diameter of the fin apertures through which the tubes are inserted. The metal components are preferably all of aluminium alloy to provide high thermal conductivity. A tool called a “bullet” is pressed down the inside length of each tube. The bullet has a diameter greater than the initial inside diameter of the tubes, so that each tube is expanded to press against the apertures of the fins. This secures the fins to the tubes with a mechanical joint. The base plate of each header has apertures for the ends of the tubes. The apertures have sufficient clearance for plastic or rubber sealing elements interposed between the metal of the tubes and base plates. A number of known ways are known to make the seal tight, for example by using a conical tool pressed into the tube ends to mechanically expand the tube ends and thereby compress the seal.
Each process has certain advantages and disadvantages as compared with the other. Heat exchangers made using the CAB process provide a higher heat exchange capacity for a given size heat exchanger and are in some ways more mechanically robust because the tubes are flattened and extend to the front and rear faces of the heat exchanger, thereby protecting the fins. A notable disadvantage is that the brazing process requires a long passage through an expensive brazing furnace. Furthermore, during operation of an engine and radiator cooling system, the radiator tubes are subject to thermal cycling (rise and fall of the temperature of the heat exchanger components) which leads to stresses as neighbouring tubes may expand to different degrees such that axial loads are imposed on tubes by their neighbours. Therefore, thermal expansion of the heat exchanger during use will not, in general, be even, and cracks can develop in certain parts of the heat exchanger depending on the pattern of the coolant flow, leading to leakage and premature failure of the heat exchanger. In particular, to maximise heat exchange capacity, the tubes are arranged side by side with the faces of neighbouring tubes opposing each other and defining a space or passage between the tubes for the fins and through which a cooling medium such as air can flow. This geometry of the tubes is, therefore, favourable as it creates a relatively large surface area over which the cooling medium can pass whilst minimising the disruption to the air flow through the heat exchanger. However, these types of header/tube combinations are prone to failure because of the stress concentrations that occur along the header/tube joint, in particular around the nose of the tubes and where the tube walls are tightly curved.
The MA process avoids the need for a costly brazing furnace, and can therefore be used to produce less expensive heat exchangers. Because the joints between the tube ends and headers are mechanical, the compression joints can be designed to allow for some longitudinal movement between the tubes and headers due to differing thermal expansion when the heat exchanger is heating up or cooling down. An all-mechanical heat exchanger therefore reduces or substantially eliminates thermal stresses between the heat exchanger components, thereby increasing heat exchanger reliability and lifetime. Such heat exchangers are, however, less efficient at transferring heat for a given size, and therefore mechanically jointed heat exchangers have to be larger to provide the same capacity. More space must therefore be provided for a larger heat exchanger in any given application. The fins, being parallel and extending to the front and back of the circular cooling fins, are also much less robust than the zigzag fins nested between flattened tubes of a heat exchanger formed using the CAB process. To maximise the heat transfer capacity, the fins are necessarily thin, about 0.1 mm in thickness, and such fins are easily deformed even by finger pressure. Any such damage will decrease flow of a cooling medium such as air through the heat exchanger. In a motor vehicle radiator, stones or grit can sometimes hit the radiator, causing cumulative damage to the cooling permeability of the matrix.
It is an object of the present invention to provide a heat exchanger and methods of manufacturing and assembling a heat exchanger which addresses at least some of these issues.